Imaging systems capable of producing multiple colors often rely upon barrier interlayers to separate adjacent imaging chemistries. In some cases, such as those in which a dye image is formed and diffused to a receiving layer, the interlayer must not only separate imaging chemistries, but must be permeable to dyes as well. Dry silver-based imageable articles are one type of imaging system that may employ interlayers in this manner.
Dry silver compositions are photothermographic compositions and contain a light-insensitive, reducible silver source; a light-sensitive silver source; and a reducing agent for the light-insensitive, reducible silver source. The light-sensitive material is generally photographic silver halide (e.g., silver chloride), which must be in catalytic proximity to the light-insensitive, reducible silver source (e.g., silver behenate). Catalytic proximity requires an intimate physical association of these two materials so that when silver specks or nuclei are generated by the irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the light-insensitive, reducible silver source by the reducing agent. It has been long understood that light exposed silver halide is a catalyst for the reduction of silver ions and the silver-generating, light-sensitive silver halide catalyst progenitor may be placed into catalytic proximity with the silver source in a number of different fashions, such as for example, by partial metathesis of the reducible silver source with a halogen-containing source and coprecipitation of silver halide and light-insensitive, reducible silver source material.
In both photographic and photothermographic emulsions, exposure of the photographic silver halide to light produces small clusters of silver atoms. The image-wise distribution of these clusters is known in the art as a latent image. As this latent image generally is not visible by ordinary means, the light-sensitive article must be further processed in order to produce a visual image. The visual image is produced by the catalytic reduction of silver ions which are in catalytic proximity to the silver halide grains bearing the latent image.
In color dry silver imaging systems, a leuco dye is typically incorporated as a reducing agent for the light-insensitive, reducible silver source, generally in combination with a spectral sensitizer for the silver halide. The leuco dye oxidizes to form a cationic dye upon development, thereby giving a colored image. In full color constructions, spectrally sensitized emulsion layers are typically coated onto a substrate and separated by one or more barrier interlayers.
Residual silver stain is a major problem with dry silver color constructions known in the art. This has been overcome by causing the developed dye image to diffuse from the dry silver layer to an image-receiving layer that is then stripped from the emulsion layer(s). In this case, a barrier interlayer must serve the dual roles of separating the chemistries of neighboring emulsion layers and allowing diffusion of the dye image under thermal processing conditions. In addition to the dye image, other components of the emulsion layers also diffuse to the image-receiving layer under thermal processing conditions. In this regard, unoxidized leuco dyes from the emulsion layers tend to diffuse to the image-receiving layer where they oxidize over time, leading to poor print stability and color separation. Therefore, there is a need for interlayers which allow dye images, but not unoxidized leuco dyes, to diffuse to the image-receiving layer.
Depending on the particular ingredients of a given dry silver layer, the development may be best carried out, for example, under acidic or basic conditions. When multiple dry silver layers with incompatible developing chemistries are employed, it is very difficult to keep development conditions within the dry silver layer from affecting the development of nearby or adjacent dry silver layers. As a result, it is advantageous to coat dry silver layers with different developing conditions on opposite sides of a transparent substrate.
U.S. Pat. No. 4,594,307 discloses a heat developable photographic material that produces a pure and stable dye image by the oxidation-reduction reaction between a reducible organic silver salt and a leuco dye reducing agent wherein the dye formed is transferred to an image-receiving layer by continuing the heating for development in order to separate the dye formed from the silver images and other residual chemicals.
The generation of color dry silver images has been accomplished using microencapsulated constructions and tri-pack (yellow/magenta/cyan) multilayer constructions such as those disclosed in U.S. Pat. Nos. 4,883,747 and 4,923,792. These patents disclose the use of polystyrene; partially hydrolyzed polyvinyl acetate; and polyvinyl butyral as barrier interlayers.
U.S. Pat. Nos. 4,021,240; 4,460,681; and 5,077,178 disclose the use of multiple image forming layers separated by barrier or interlayers.